Endoscopy in the medical field allows internal features of the body of a patient to be viewed without the use of traditional, fully-invasive surgery. Endoscopic imaging systems enable a user to view a surgical site and endoscopic cutting tools enable non-invasive surgery at the site. For instance, an RF generator provides energy to a distal end tip of an RF probe within the surgical site. In one mode, the RF probe provides RF energy at a power level to ablate or otherwise surgically remove tissue. In another instance, RF energy is provided to the RF probe in order to coagulate the tissue at the surgical site to minimize bleeding thereat.
Tissue ablation is achieved when a high power electrical signal having a sufficiently large voltage is generated by the control console and directed to the attached probe. Application of the high power signal to the probe results in a large voltage difference between the two electrodes located at the tip of the probe (presuming a bipolar probe), with the active electrode being generally 200 volts more than the passive or return electrode. This large voltage difference leads to the formation of an ionized region between the two electrodes, establishing a high energy field at the tip of the probe. Applying the tip of the probe to organic tissue leads to a rapid rise in the internal temperature of the cells making up the neighboring tissue. This rapid rise in temperature nearly instantaneously causes the intracellular water to boil and the cells to burst and vaporize, a process otherwise known as tissue ablation. An electrosurgical “cut” is thus made by the path of disrupted cells that are ablated by the extremely hot, high energy ionized region maintained at the tip of the probe. An added benefit of electrosurgical cuts is that they cause relatively little bleeding, which is the result of dissipation of heat to the tissue at the margins of the cut that produces a zone of coagulation along the cut edge.
In contrast to tissue ablation, the application of a low power electrical signal having a relatively low voltage to the active electrode located at the tip of the probe results in coagulation. Specifically, the lower voltage difference established between the active and return electrodes results in a relatively slow heating of the cells, which in turn causes desiccation or dehydration of the tissue without causing the cells to burst.
Basic operation of an electrosurgical system can be analyzed in view of at least two relationships.
The first relationship is described by Ohm's law, which in simplest terms, is represented by the equation V=IxR or alternatively V=(IxZ), where:
I=electrical current;
R=resistance or impedance to the current (hereafter referred to as Impedance (Z), which includes capacitive and inductive loading); and
V=voltage or force that “pushes” the current through the impedance.
The second relationship is the definition of power (P), which can be calculated by the equation (P=IxV). The resultant product of current I and voltage V represents the amount of energy that is transferred within a defined period of time.
FIGS. 1 and 2 correspond to FIGS. 1 and 2 of U.S. Patent Publication 2007/0167941, the disclosure of which is hereby incorporated by reference.
As illustrated in FIG. 1, a typical electrosurgical system 10 includes an electrosurgical probe 12 (hereafter referred to simply as “probe”) and a control console or controller 14. The probe 12 generally comprises an elongated shaft 16 with a handle 18 at one end and a tip 20 at the opposite end. A single active electrode 19 is provided at the tip 20 if the probe 12 is of a “monopolar” design. Conversely, the probe 12 may be provided with both an active electrode 19 and a return electrode 21 at the tip 20 if the probe is “bipolar” in design. The probe 12 connects to control console 14 by means of a detachable cable 22. The current for energizing the probe 12 comes from control console 14. When actuated, the control console 14 generates a power signal suitable for applying across the electrode(s) located at the tip 20 of the probe 12. Specifically, current generated by the control console 14 travels through the cable 22 and down the shaft 16 to tip 20, where the current subsequently energizes the active electrode 19. If the probe 12 is monopolar, the current will depart from tip 20 and travel through the patient's body to a remote return electrode, such as a grounding pad. If the probe 12 is bipolar, the current will primarily pass from the active electrode 19 located at tip 20 to the return electrode 21, also located at tip 20, and subsequently along a return path back up the shaft 16 and through the detachable cable 22 to the control console 14.
Configuration of the control console 14 is carried out by means of an interface 15, while actuation and control of the probe 12 by the surgeon is accomplished by one or more switches 23, typically located on the probe 12. One or more remote controllers, such as, for example, a footswitch 24 having additional switches 26 and 28, respectively, may also be utilized to provide the surgeon with greater control over the system 10. In response to the surgeon's manipulation of the various switches on the probe 12 and/or remote footswitch 24, the control console 14 generates and applies either a low power signal or high power signal to probe 12. As will be discussed in greater detail below, application of a low power signal to probe 12 results in coagulation of the tissue adjacent the tip 20 of the probe 12. In contrast, application of a high energy signal to probe 12 results in tissue ablation.
While operating in coagulation mode, the control console 14 of the prior art system shown in FIG. 1 is configured to drive the attached probe at a low, but constant, power level. Due to inherent varying conditions in tissue (i.e., the presence of connective tissue verses fatty tissue, as well as the presence or absence of saline solution), the impedance or load that the system experiences may vary. According to Ohm's law, a change in impedance will result in a change in current levels and/or a change in voltage levels, which in turn, will result in changing power levels. If the operating power level of the system changes by more than a predefined amount, the control console will attempt to compensate and return the power back to its originally designated level by regulating either the voltage and/or current of the power signal being generated by the console and used to drive the attached probe.
While operating in tissue ablation mode, the control console of the system shown in FIG. 1 is configured to drive the attached probe at as high a power level as possible without exceeding a maximum average power level, which in some instances may equal 400 watts.
The electrosurgical system shown in FIG. 1 modulates the entire power supply signal as a whole, turning the signal on and off in a manner similar to a pulse width modulated (PWM) signal. Furthermore, the power signal is dynamically modulated on and off so as to behave like a PWM signal having a variable duty cycle. As a result, the percentage of time that the power signal is “on”, compared to the percentage of time that the signal is “off”, will vary depending on the percentage of time that the power levels of the signal exceed the maximum limit over a predetermined interval of time.
Consequently, the duty cycle of the power signal is dynamically modulated so that even though the power levels of the signal may briefly exceed the maximum power limit for a portion of time during a specified interval, the average power level over that interval of time remains acceptable.
To further illustrate the above point, FIG. 2 depicts several examples of high frequency power signals generated by the control console 14 over a 20 millisecond period of time and used to drive the attached probe 12. Signal A is a power signal in the form of a 200 KHz sine wave. No modulation of signal A is present with respect to a signal duty cycle, resulting in a power signal that is continuously on (i.e., 100% duty cycle) for the entire 20 millisecond duration.
In FIG. 2, signal B is similar to signal A, but has been briefly modulated roughly half-way through the 20 millisecond period. In this instance, for example, changing environmental variables may have resulted in the power level of the signal briefly exceeding an established maximum limit during the previous 20 millisecond period (not shown). To compensate for this prior spike in power level and assure that the average power of the signal does not exceed a maximum limit, the system briefly modulates signal B during the next 20 millisecond period (shown), effectively turning the signal off for a moment. Thus, for example, signal B is modulated or turned off for approximately 5 milliseconds during the 20 millisecond period depicted, resulting in the signal effectively having a 75% duty cycle for the period shown.
To compensate for power level spikes that are larger in magnitude or longer in duration, the system dynamically modulates the duty cycle of the power signal during the next monitoring interval to effectively turn off the signal for a longer period of time. For example, signal C of FIG. 2 is similar to signal B, but is modulated to have a lower duty cycle, resulting in signal C being turned off for a longer period of time during the 20 millisecond interval shown.
By dynamically adjusting a duty cycle of the power signal, the average power of the signal can be maintained below an established maximum power limit. Furthermore, it has been observed that the ionized high energy field maintained at the tip of the probe 12 does not collapse, but remains stable, if the effective duty cycle of the power signal is modulated quickly enough (i.e., turning the signal on or off in increments of 50 milliseconds over a 1 second period).
In the electrosurgical system 10 illustrated in FIG. 1 above, the duty cycle is varied for the waveform only in instances where the voltage or current causes the power value of the RF probe to exceed the acceptable power value. Thus, in the prior art, the duty cycle is varied by differing amounts, as necessary, to account for unintended increases in power value beyond the average power value of the system.
A non-volatile memory device (not shown) and reader/writer (not shown) can be incorporated into the body 18 of the probe 12, or alternatively, incorporated into or on the cable 22 that is part of the attachable probe and which is used to connect the probe 12 to the control console 14 of the system. Alternatively, the memory device may be configured so as to be incorporated into or on the communication port that is located at the free end of the cable 22 and which is used to interface the cable with a corresponding port on the controller 14.
During manufacturing of the attachable probe shown in FIG. 1, data representing probe-specific operating parameters is loaded into the memory device. Upon connection of the attachable probe 12 to the control console 14 of the system 10, the data stored in the probe's non-volatile memory can be accessed by the reader and forwarded on to the controller 14. As such, once a probe 12 is connected, the controller 14 accesses the configuration data of the specific probe 12 and automatically configures itself based on the operating parameters of the probe 12.
Beyond probe-specific operating parameters, the prior art memory device within each attachable probe 12 can store additional data concerning usage of the probe 12. This usage data can comprise a variety of information. For example, usage data may represent the number of times a probe 12 has been used, or the duration of the time that the probe has been activated overall or at different power levels. Additional usage data may restrict the amount of time that a specific attachable probe can be used. Alternatively, a probe 12 may be programmed so it can only be used for a limited duration of time starting from the moment the probe was first attached to a control console and powered up. For example, a probe may be programmed to that it only functions for a 24-hour period starting from when the probe is first activated. Based on a clock maintained within the control console, a time stamp is written to the memory device of the probe when the probe is attached to the console for the first time and powered up. Any later attempted use of that probe will trigger a comparison of the stored time stamp to the current time reported by the control console, and if the allotted amount of time has already passed, the system will not allow the probe to be used.
Alternatively, a specific prior art probe is dynamically restricted, so that the overall amount of time allocated for use of the probe will vary depending not only on the amount of time the probe has been used, but also the power levels that the probe was driven at during its use. As such, a specific attachable probe may be limited to 1 hour of use if always driven at a maximum power, but may be usable for 3 hours if all prior uses occurred at substantially lower power levels.
In addition to usage data, the prior art memory device can store information concerning any errors that were encountered during use of the probe 12. For example, the failure of a probe to activate would lead the control console 14 to issue and store one or more error codes into the probe memory. Technicians can later retrieve these error codes to aid in their examination of the failure.
In addition to probe-specific operating parameters and usage data, the memory device incorporated into each probe may also be programmed by the manufacturer to include software scripts or updates for the control console of the system.
In the electrosurgical system illustrated in FIG. 1, the power output from control console 14 has a constant source impedance regardless of the probe utilized or the mode of operation.
As discussed above, the electrosurgical system 10 shown in FIG. 1 provides a modulated duty cycle power output only to decrease power output in instances where the power exceeds the desired power due to incidental variations in the impedance of the load or other power control issues. Further, the duty cycle value varies depending on the amount that the power exceeds the desired average power level. Thus, during normal operation, the output power value may, in some instances, not exceed the desired intended constant average power value resulting in no duty cycle variations in the power output by an energy generator.
The present invention is directed to improving cutting or coagulation of tissue by an RF probe, such as by optimizing power delivery to tissue by adjusting the source impedance value of an RF generator.
In one embodiment of the invention, information regarding source impedance values for an RF generator is stored on an RF probe and read by a processing device. The processing device controls the source impedance value of the RF generator based on the stored values to optimize power transfer from the RF generator to tissue via the RF probe.
In another embodiment of the invention, improved operation of an electrosurgical system is obtained by duty cycling of voltage output from a RF generator to increase the instantaneous voltage value applied to an RF probe. The duty cycling information is read from a memory device on the RF probe. Modulating the RF voltage value at a secondary frequency with a duty cycle of less than 100% reinitiates a voltage arc dynamically on different tissues at the beginning of each time period that includes the duty cycle value. Periodically reinitiating arcing by duty cycling the RF output voltage value helps to maintain consistent burn characteristics on various tissues. Also, constant duty cycling tends to physically push tissue away from the probe tip during ablation to maintain good spacing between the probe tip and tissue, which creates optimal arcing, and thus helps to prevent clogging. Further, the duty cycling of output voltage helps to control depth of necrosis because the heated tissue is allowed to thermally relax between each duty cycle application of RF voltage.
Another advantage of the invention is that the duty cycle applied to the RF voltage output decreases the amount of total time the probe is exposed to high RF voltage, which reduces probe degradation as compared to a continuous application of RF power. In this embodiment, cyclically applying voltage to the RF probe at less than the maximum allowable voltage value, or even at the same voltage value as a non-duty cycled RF generator output, reduces heating of the surgical site, such as a joint, without significantly affecting cutting performance.